1. Field of the Invention
This invention relates to a fuel cell system. In particular, it relates to a fuel cell system mounted to an automobile.
2. Related Art
Recently, fuel cell systems have drawn attention as a new power source for automobiles. The fuel cell system includes, for example, a fuel cell that generates electric power by chemically reacting reactant gas, a reactant gas supplying device that supplies a reactant gas to the fuel cell via the reactant gas channel and a control device that controls the reactant gas supplying device.
The fuel cell has, for example, a stack structure in which several tens to several hundreds of cell are laminated. Each cell is configured by sandwiching a membrane electrode assembly (MEA) by a pair of plates. The membrane electrode assembly is configured by an anode and a cathode and a polymer electrolyte membrane which is arranged between the two electrodes.
The electric power generation occurs as the result of an electrochemical reaction by supplying hydrogen gas as the reactant gas to the anode and air including oxygen (oxidant gas) as the reactant gas to the cathode.
Incidentally, it is known that when cell voltage decreases to a value lower than a specified value (called voltage lower limit herein after), it is in a degradation range in which the degradation of MEA is accelerated. To solve the problem, for example, a system is proposed to suppress the voltage decrease by limiting current and power output, etc. (see Japanese Patent application publication 2004-172055 A, hereinafter referred to as Patent Document 1).
More specifically, in the system disclosed in Patent Document 1, the threshold value A and the threshold value B, which is lower than the threshold value A, are set for the cell voltage, and when the lowest cell voltage is lower than the threshold value A, the output is limited in order for the lowest cell voltage not to become lower than the threshold value B. Furthermore, when the lowest cell voltage becomes lower than the threshold value B, the output is farther limited according to the variation of the lowest cell.
However, it has been confirmed that the voltage lower limit varies according to fuel cell temperature. Especially in the low temperature range, there is a tendency for the voltage lower limit to be higher, the lower the temperature becomes. However, the threshold value of the voltage lower limit that is disclosed by the Patent Document 1 does not depend on the temperature of the fuel cell and was constant.
Therefore, for example, when the fuel cell is driven in the lower temperature state, the voltage lower limit becomes higher than the specified threshold value and there is a danger of entering the degradation range, despite the cell voltage being higher than the threshold value. When the threshold value is set high, the voltage lower limit may become largely lower than the threshold value so that the MEA is protected excessively, thereby degrading the drivability.
As described above, it was difficult to keep the compatibility between the merchantability and the protection of the MEA.